Composite cell bottom for aluminum electrowinning

ABSTRACT

A cell for the electrowinning of aluminum from molten salts has a cell bottom lining consisting partly of a refractory mass (4) and partly of carbon bodies (5). At least 30% and preferably 50% or more of the cell bottom area is occupied by the refractory mass (4). The carbon bodies (5) are level with the refractory mass (4) or are recessed therein.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of prior applicationPCT/US87/02357 filed Sep. 16, 1987.

TECHNICAL FIELD

The invention relates to aluminum reduction cells of the type having acell bottom comprising a carbon body through which current is suppliedto a pool of molten aluminum resting on the cell bottom, as well as tomethods of fabricating and assembling such cells and methods ofproducing aluminum by electrolysis of a molten salt containing adissolved aluminum compound in particular molten cryolite containingalumina, using an improved cell of this type.

BACKGROUND ART

Conventional Hall-Heroult cells for the electrolytic production ofaluminum employ a carbon cell bottom which serves to supply current to adeep pool of molten aluminum forming the cathode. The cathodic aluminumis necessarily thick (at least 80-100 mm) because carbon is non-wettableby molten aluminum and during operation would not completely cover thecarbon if the aluminum layer were thinner. In the conventionalarrangement, a horizontal steel conductor bar is embedded in the lowerpart of the carbon cell bottom for the supply of current from anexternal source. Thus, the entire cell bottom in contact with the moltenaluminum cathode consists of carbon which, in operation, is impregnatedwith sodium species and other ingredients of the cryolite leading to theformation of toxic compounds including cyanides. Despite the manydisadvantages associated with carbon as cathode current feeder material(non-wettability by aluminum, necessitating deep pool operation; therelatively high electrical resistance of carbon, leading to energylosses; reactions within the cell environment necessitating disposal oflarge quantities of contaminated carbon when the cell bottom is renewed;swelling, which must be compensated by supporting the cell sidewalls incradles, etc), attempts to replace carbon with theoretically moreadvantageous materials and employing new cell designs have not so farmet with success.

Thus, for example, an aluminum production cell having an electricallynon-conductive refractory lining with a "bottom entry" current collectoris described in U.S. Pat. No. 3,287,247. The inner end of the currentcollector has a cap of TiB₂ projecting into a depression containing adeep pool of molten aluminum. U.S. Pat. No. 3,321,392 describes asimilar arrangement in which the protruding ends of TiB₂ conductor barsare rounded. U.S. Pat. Nos. 3,093,570 and 3,457,158 disclose similardesigns in which bottom-entry cylindrical current collector bars orposts of TiB₂ or graphite extend through a non-conductive refractorylining consisting throughout of powders of alumina and cryolite oraluminum fluoride.

U.S. Pat. No. 4,613,418 has proposed an aluminum production cell with analumina potlining in which bottom-entry current collectors are embeddedand extend to a recess in the potlining. To prevent the unwantedcollection of sludge in these depressions, this patent proposes fillingthe depressions with balls of aluminum-wettable material. Relateddesigns are proposed in U.S. Pat. No. 4,612,103.

These alternative cell designs, using a non-carbon cell bottom, havegreat promise. Replacement of the carbon cell bottom with, e.g., aluminaleads to potential savings in materials and operating costs. However,such proposals heretofore have generally relied on the use of a familyof materials known as Refractory Hard Metals ("RHM") encompassing theborides and carbides of metals of Group IVB (Ti, Zr, Hf) and VB (V, Nb,Ta) of the periodic table of the elements. TiB₂ has been identified asthe most promising RHM material. The use of these materials as part ofthe current supply arrangement has encountered a number of problemsincluding cost and the difficulty of producing and machining largepieces of the materials. Such difficulties have led to the designexpedients proposed in the aforementioned U.S. Pat. Nos. 4,613,418 and4,612,103, where, for example, small pieces of TiB₂ are assembled orpacked together in an environment of molten aluminum as part of thecurrent supply arrangement.

The problems experienced with RHM current collectors and furtherexpedients for dealing with them, namely the provision of a protectivebarrier incorporating a molten fluoride- or chloride-containing saltmixture or a getter such as particulate aluminum, are further describedin EP-A-0 215 555.

In addition to the problems associated with the use of RHM materials,the cell design employing multiple current collector bars or posts ofrelatively small cross-section penetrating through the cell lining hasmany inherent drawbacks since each current collector must carry a highcurrent and the failure of any single current collector can lead to atotal cell failure.

A number of proposals have been made for alternative cell designs havingcarbon cell bottoms in conjunction with inert materials underneathand/or at the sides of the carbon. See, for example, U.S. Pat. Nos.3,390,071, 4,592,820, 4,673,481 and 4,619,750. Side-entry current feederdesigns have also been proposed, e.g. in U.S. Pat. No. 3,370,071, butsuch designs have not found acceptance on account of a number ofinherent drawbacks. There has been also a proposal in UK-A-2 076 021 toprovide dividers of insulating material that subdivide the liquidaluminum cathode so that its effective surface area is somewhat lessthan that of facing dimensionally stable anodes, with a view toimproving the anode lifetime. This arrangement, however, complicates thecell bottom and adds to its cost.

UK-A-1 206 604 has disclosed carbon blocks which protrude above a celllining for the purpose of collecting sludge on the cell bottom. Thisdesign is, however, confined to deep pool operation and the protrudingcarbon elevations are subject to erosion.

The problems associated with replacing the carbon bottoms of aluminumreduction cells have thus not been resolved in a satisfactory manner, sothat carbon cell bottoms continue to be the industry standard.

DISCLOSURE OF INVENTION

This invention is based on the realization that considerable savings canbe made and other advantages obtained by replacing substantial portionsof the carbon in the cell bottom by refractory materials in areas wherethe carbon was considered necessary to provide for an adequate supply ofcurrent to the pool of aluminum forming the cathode.

The invention therefore provides a cell for the electrowinning ofaluminum from molten salts utilizing carbon cathodes, in which the cellbottom lining consists partly of a refractory mass and partly of carbon,the total upwardly facing surface area of the carbon cathode under theanode being smaller than the horizontal surface area of the anode.

In this description: "projected anode area" or "horizontal surface areaof the anodes" mean the surface area of the cell bottom defined by aline bounding the periphery of each anode projected onto the cellbottom. Also, it is understood that the term "carbon cathode" means thecarbon cathode current feeder, since the carbon acts to supply currentto the pool of molten aluminum which forms the effective cathode in thecell.

According to the invention, a cell for the electrowinning of aluminumfrom molten salts of the type having a plurality of anodes disposed overa cell bottom comprises a carbon cathode through which current issupplied to a pool of molten aluminum on the cell bottom, and ischaracterized in that the cell bottom is lined with at least one body ofcarbon and at least one mass of non-conductive, refractory materialjuxtaposed with the carbon body or bodies to make up a composite cellbottom composted of adjacent areas of current-conducting carbon andnon-conducting refractory material, the upper surfaces of the carbonareas being located at the same level as or lower then the uppersurfaces of the refractory areas, and the total upwardly facing surfacearea of the carbon in the cell bottom located under the anodes beingsmaller than the horizontal surface area of the anodes.

In these cells, at least a part of the surface area of the anodesprojected on the cell bottom thus covers areas of the non-conductingrefractory material. Typically, 20% or more of the projected anode areawill be occupied by the refractory material and in some embodiments theentire anode surface area projected onto the cell bottom is occupied bythe refractory material. This is possible by locating the carboncathodes so that they provide an adequate distribution of current to thecathodic pool of molten aluminum. The pool of aluminum itself is such agood electrical conductor that current is evenly distributed at thesurface of the pool. By thus replacing a substantial fraction of thecarbon (as compared to a conventional carbon cell bottoms) considerableadvantages are achieved, including:

initial materials cost saving of the amount of block carbon required.

a considerable reduction of the amount of contaminated carbon to bedisposed of when the cell bottom is renewed, this contaminated carbonbeing a non-reusable hazardous waste.

possible capital saving by reducing the production of the block carbon.

by eliminating carbon pastes currently used to cement the carbon blocks,exposure of the workers to the fumes is eliminated.

longer average cell life because of reduction of the swelling of thecarbon blocks and replacement by a non-swelling refractory material.

because of this reduction of swelling, there is less pressure on thesides of the cell. Consequently, the number of cradles or other devicesto support the cell sidewalls is reduced, further simplifyingconstruction of the cell and enabling a significant capital saving.

when the cell is shut down and re-rebuilt, the alumina or otherrefractory material can be ground and re-used. This entails asignificant saving in raw materials (cryolite) absorbed by therefractory because these materials can now be recycled whereas withall-carbon cell bottoms, such absorbed materials are lost with thehazardous waste.

reduction of the manpower and time to construct or re-line a cell.

Furthermore, this new composite cell bottom is relatively inexpensive,easy to construct, composed of tried and tested materials whoseperformance in the cell environment is known, and suitable for retrofitof existing cells but can also be applied to new cell designs. In thesecells, preferably at least 30% and often 50% or more of the surface areaof the carbon cell bottom lining is replaced by a refractory mass.Usually, no more than 80 or at most 90% of the surface area of the cellbottom is made up of the refractory mass, depending on the geometricalconfiguration. Also, as a general rule, the upwardly-facing carboncathode area will be less than 50% of the active anode surface area,i.e., its horizontal area plus the operative area of the sides.

In most embodiments, the refractory mass extends to the cell sides. Therefractory mass advantageously comprises tabular alumina, for example itmay be a mixture or layers of tabular alumina and alpha alumina asdisclosed in EP-A-0 215 590, but may also consist at least in part offused alumina, e.g., slabs of fused alumina forming the upwardly-facingsurface of the cell bottom. The upper surface of the refractory mass maybe wettable by molten aluminum, e.g., by incorporating aluminum-wettableRHM materials.

The level of the refractory mass, i.e., its upper surface, can be at thesame level as the surface of the carbon cathode. In preferredembodiments, however, the level of the refractory mass is higher thanthe level of the carbon cathode. In this way, the depth of the pool ofmolten aluminum above the refractory mass can be reduced whilemaintaining this level sufficiently above the carbon cathode to protectthe carbon from contact with the electrolyte during fluctuations of thepool level. Thus, when the level of the carbon cathode is below thelevel of the refractory mass, this permits a shallow aluminum pool abovethe refractory mass thereby reducing the fluctuation of the moltenaluminum. This in turn permits the electrode gap to be reduced thanks toreduced fluctuation of the molten pool.

The carbon cathode can consist of a plurality of sections usually ofrectangular shape (in order to reduce the effect of the magnetic fieldand the fluctuation of the molten aluminum pool). These carbon cathodesections are longitudinal in the cell, or transversal. Alternatively,the carbon cathode sections in the cell are placed under the anodes andare of rectangular, round or of any convenient shape. In someembodiments, the carbon cathode sections in the cells are not placed incorrespondence of the anodes and are of rectangular or round or of anyother convenient shape. One particularly advantageous configurationwhich will be described later consists of a chequer pattern.

The areas of carbon may be rectangular (in plan view) and the refractorymass can occupy a space made up by multiples of the rectangular spacescorresponding to the carbon cathodes. Usually both the carbon and therefractory mass extend down to the cell lining or other support surfaceof the cell bottom, but this is not necessary and in some embodimentsthe carbon bodies may extend only part of the way down and be supportedin a recess in the refractory mass.

Electrical contact of the carbon cathode to the external bus bars can bemade through conventional horizontal collector bars, i.e., usuallytransverse in relation to the cell, but other arrangements are possiblein new cell designs.

Vertical pins, plates or bars of metals resisting the operatingtemperature of the cell may be inserted in the carbon cathode andconnected to the collector bars, so reducing the electrical resistivityof the carbon bodies. Such pins, plates or bars may alternatively beconnected to the conductive outer shell of the cell and from there tothe bus bars.

The surface of the carbon cathodes in contact with the molten aluminummay also be increased by providing cuts, holes, slots or other recessesin the carbon body extending vertically but not reaching the currentcollecting means and filled with aluminum. Furthermore, spacings (slots)can be provided between the carbon cathodes and the adjacent refractorymass, these spacings or slots extending vertically and being filled withaluminum, but not reaching the current collecting means.

A feature of the described cells is that the cell bottom contains noportions of carbon which are not in contact with the molten aluminum. Inthe cells according to the invention, all the carbon serves as currentfeeder. There is no carbon which serves merely as a cell lining.

The cell according to the invention can operate with conventionalpre-baked carbon anodes or with oxygen evolving anodes, such asdimensionally stable anodes having a cerium oxide-fluoride surfacecoating.

A method of fabricating or renovating (retrofitting) an aluminumproduction cell bottom according to the invention consists of lining thecell bottom with a refractory mass and carbon, the total upwardly-facingsurface area of the carbon cathode located under the anode locationsbeing smaller than the projection of the horizontal area of the anodesto be fitted in the cell, the upper surfaces of the carbon blocks beinglocated at the same level as or lower than the upper level of the massof refractory material. Advantageously, the carbon may be blocks of thesame shape and size as the rows of carbon blocks in an existing cell,certain of these blocks being replaced in a retrofit operation with amass of refractory material such as alumina.

The invention also relates to the production of aluminum e.g. by theelectrolysis of alumina in molten cryolite, using the improved cell asdescribed herein.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be further explained with reference to theaccompanying schematic drawings, in which:

FIG. 1 is a transverse cross-section through an aluminum electrowinningcell showing different forms of the cell bottom according to theinvention;

FIGS. 2-3 are views similar to FIG. 1 illustrating further forms of thecell bottom;

FIG. 4 is a diagrammatic plan view of one form of a cell bottom as shownin FIGS. 1-3;

FIGS. 5A, 5B and 5C are views similar to FIG. 4 showing different cellbottoms;

FIGS. 6A-6F are diagrammatic plan views of other cell bottomconfigurations; and

FIG. 7 is a longitudinal cross-sectional view through part of anothercell.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a transverse cross-section through a Hall-Heroult cell ofgenerally traditional design except that it has been retrofitted with animproved cell bottom according to the invention. The cell comprises aheat insulating shell 1, 2 having transverse cathode current-feeder bars3 for example of steel or other suitable high-temperature resistantalloy. This shell 1, 2 contains a cell bottom made up of a mass 4 orcompacted inert refractory material such as alumina and carbon bodies 5.The current-feeder bars 3 pass through the carbon bodies 5 for thesupply of electric current to a pool 6 of molten aluminum resting on thetop surface of the cell bottom. On top of the molten aluminum pool 6 isa layer of molten electrolyte 7, for example cryolite containing up toabout 10% of alumina at a temperature of about 900°-970° C. Theelectrolyte 7 is surrounded by a freeze 8 of solidified electrolytewhich covers the top edges of the refractory mass 4 and also extendsaround the periphery of the molten aluminum pool 6. Into the electrolyte7 dip two rows of pre-baked carbon anodes 9 suspended by a conventionalanode suspension arrangement (not shown).

In traditional Hall-Heroult cells, the cell bottom (i.e., correspondingto parts 4 and 5) is composed substantially entirely of carbon. Theimproved cell, as shown, having a mass 4 of refractory material makingup a major part of the cell bottom, can conveniently be constructed as aretrofit operation when the existing carbon cell bottom must bereplaced.

The carbon bodies 5 shown in FIG. 1 lie under the anodes 9 but theupwardly-facing surface area of the carbon bodies 5 under the anodes isless than the projected area of the anodes 9. Various configurations ofhow the bodies 5 may be disposed in the cell bottom and how the anodes 9project onto the top face of the cell bottom will be described later.

FIG. 1 shows two different arrangements for the upper faces of bodies 5.The left-hand body 5 has a flat top face 10 flush with the flat top faceof the refractory mass 4, thus making up a flat uninterrupted cellbottom covered by the molten aluminum poll 6. The right-hand body 5 hastwo slots 11 machined into its upper face and extending down to withinseveral centimeters of the current-feeder bars 3. These slots 11 aremade wide enough so that they fill u p with molten aluminum from thepool 6. A single slot 11, or more than two slots could be provided, asconvenient, or instead of slots there could be recesses of any othersuitable shape, e.g., with a round cross-section. The purpose of theseslots or other recesses is to reduce the current carrying path betweenthe bars 3 and the aluminum pool 6, thereby avoiding energy loss the dueto the relatively low electrical conductivity of carbon. It isunderstood that all of the carbon bodies 5 in the cell bottom willusually be identical, i.e., all as shown int he left of FIG. 1 or all asshown in the right of FIG. 2. The same comment applies also to FIG. 2.

For convenience, in the remaining Figures, like reference numeralsdesignate the same parts as in FIG. 1.

The cell shown in FIG. 2 is the same as that shown in FIG. 1 except fordetails of the current supply arrangement for the carbon bodies 5.Adjacent the left hand carbon block 5 are channels 12 in the refractorymass 4. These channels 12 end several centimeters above thecurrent-collector bars 3 and are filled with molten aluminum from thepool 6. Again, this serves to reduce the current-carrying path betweenthe bars 3 and pool 6. Conveniently, the walls of the mass 4 formingchannels 12 may be lined with an aluminum-wettable material such as TiB₂or a composite containing TiB₂.

The right-hand part of FIG. 2 shows a carbon block 5 incorporating aseries of plates or posts 13 upstanding on the bars 3. The bars 3 andposts 13 may both be of steel or a weldable alloy such as NiAl, andjoined by welding. These plates or posts 13 extend upwardly in theblocks 5 but stop several centimeters short of their upper faces. Anyconvenient number of plates or posts 13 can be provided. This is thusanother way of reducing the current-carrying path through the carbon ofblocks 5.

Various combinations can be made of the features shown in FIGS. 1 and 2.For example, the plates or posts 13 can be combined with externalchannels 12; or the external channels 12 can be combined with slots 11.

In the cell illustrated in FIG. 3, the carbon bodies 5 are located inrecesses 14 in the cell bottom so that the top face 10 of bodies 5 isbelow the top 15 of the refractory mass 4, which has bevelled edgesextending down to the top face 10 of bodies 5. By using thisarrangement, it is possible to lower the upper level of the pool 6 and,in turn, reduce the gap between the anodes 9 and pool 6.

FIG. 4 is a schematic plan view showing one possible arrangement of howthe anodes 9 are disposed over the central flat part of the cell bottommade up of the refractory mass 4 and carbon bodies 5. For convenience,optional features such as the slots 11, channels 12 and recesses 14 arenot shown. The current-collector bars 3 which protrude laterally fromthe cell are also not shown. The anodes 9 are represented in outline,i.e., as projected onto the cell bottom. FIG. 4 shows carbon bodies 5extending as two side-by-side longitudinal strips along the cell andlocated under the two rows of anodes 9. These anodes 9 have the sameshape, dimensions and location as in a conventional cell. The projectionof each anode 9 on the cell bottom extends in part over the refractorymass 4 which occupies a major part of the cell bottom area. In thisparticular embodiment, the carbon bodies 5 are located partly under theanode projections 9.

FIGS. 5A, 5B and 5C show three different configurations in which thecarbon bodies 5 also extend partly under each anode projection.

In FIG. 5A, transverse carbon bodies 5 are located under eachside-by-side pair of anodes 9. In FIG. 5B, a rectangular or squarecarbon body 5 is located centrally under a cluster of four anodes 9. InFIG. 5C, a single carbon body 5 is located centrally under each anode 9;two of these bodies 5' are shown as square and two others 5" of circularshape. However other shapes are possible. As for the other embodiments,the anodes 9 project onto the refractory mass 4. In the illustratedexamples, the refractory mass 4 occupies approximately the followingpercentages of the projected anode area: 47% in FIG. 4, 51% in FIG. 5A,76% in FIGS. 5B and 70%/66% in FIG. 5C

FIGS. 6A-6F are schematic diagrams of the cell bottom shown subdividedinto rectangles each representing the location of a carbon block 5 in aconventional cell bottom to be replaced. In the conventional procedure,the carbon blocks 5 are bonded at their interfaces by carbon pasteswhich release hazardous fumes. By reducing the number of theseinterfaces, and in some cases even by eliminating them, an importantadvantage is obtained. For convenience, these interface lines are shownin FIGS. 6A-6F even at the locations occupied by a monolithic refractorymass, e.g., of packed alumina.

FIGS. 6A-6D illustrate a cell bottom previously made up of rows of fourrectangular carbon blocks 5 and in which some of the carbon blocks havebeen replaced. Typically each transverse row of four carbon blocks isassociated with a transverse current feeder bar (not shown), like thebar 3 on FIG. 1. In the retrofitted cell bottom of the invention shownin FIG. 6A, all of the carbon blocks along the sides and ends of thecell are replaced by a refractory mass 4. This leaves a centrallongitudinal cathode made up of carbon bodies 5.

The arrangement shown in FIG. 6B is similar to that in FIG. 6A, exceptthat only the lateral carbon bodies are replaced with the refractorymass 4, so that the carbon cathode made of bodies 5 extends fromend-to-end of the cell.

FIG. 6C shows an inverse a arrangement where the carbon bodies 5 arearranged around the periphery of the cell bottom, leaving a rectangularcentral opening filled with the refractory mass 4.

FIG. 6D shows how substantially square cathodes can be made up (cf. FIG.5B); in this example, the surface area of the carbon block 5 is lessthan 1/4 of the cell bottom area.

FIGS. 6E and 6F show further cell bottom configurations possible forretrofitting a cell made up of rows of five carbon blocks. FIG. 6E showsa checkerboard design obtained by replacing alternate carbon blocks 5 bythe refractory mass 4. This design has two significant advantages.Firstly, a very uniform current distribution can be obtained using allof the existing cathode current connector bars. Secondly, there are nointerfaces between the carbon blocks thereby eliminating the need forbonding with carbon paste.

FIG. 6F shows a similar checker arrangement in which even more carbon isreplaced, i.e., around the periphery of the cell bottom.

Obviously, many more designs are possible for the cell bottom, dependingon the size and shape of the carbon blocks for any given cell bottom.Also, in FIGS. 6A-6F the locations of the anodes are not shown. It isevident that the cell bottom configuration can be set up as a functionof a given anode configuration (rows of one, two or three anodes) ifdesired.

For a retrofit operation, it is clearly advantageous to design a cellbottom based on the dimensions of the existing carbon blocks. In thisway, the existing production line for the carbon blocks can be usedwithout modification. In some cases it may however be advantageous touse smaller carbon blocks, either using a modified production line or bycutting the blocks in halves, or quarters, etc.

The cell bottoms illustrated in FIGS. 5A-5C and 6A-6F may have a planartop face, i.e., with the top of the carbon blocks 5 flush with the topof the refractory mass 4. This arrangement is particularly suitable foroperation with a deep pool of molten aluminum. Alternatively, foroperation with a deep pool or a relatively shallow pool of moltenaluminum, the top surface of the refractory mass 4 can be made wettableby molten aluminum, e.g., by incorporating RHM materials, and the carbonblocks 5 can be recessed so that their top surfaces are below thealuminum-wettable top surface of the refractory mass 4. In this way,thee are deeper pools of molten aluminum over the carbon bodies 5,sufficiently deep to protect the carbon bodies from attack by theelectrolyte, e.g., during fluctuation of the level of the pool of moltenaluminum. This recessed or stepped configuration is also veryadvantageous in that by having confined in the aluminum pool are damped,permitting operation with a narrow gap between the anodes and thealuminum pool. These recessed embodiments may advantageously employtiles or slabs of fused aluminum containing RHM inclusions in theirsurface, as described in copending application now U.S. Pat. No.5,004,524 and as illustrated in FIG. 7.

FIG. 7 is a longitudinal cross-section through part of another aluminumelectrowinning cell employing carbon bodies in the form of bars 5 in arecessed shallow-pool configuration. The cell has a conductive baseplate 33 e.g. of steel to which the bars 5 are connected by steel orother alloy plates or posts 43 having slots 44 in their upper ends toaccommodate for expansion. In this example, the bars 5 do not extendright down to the base plate 33 but are contained in recesses in therefractory mass 4. On top of the alumina or other refractory mass 4 areblocks 34 of refractory material having an upper layer 35 of RHM, forexample TiB₂ particles or lumps embedded in a layer of tabular aluminaor in fused alumina as described in greater detail in copendingapplication now U.S. Pat. No. 5,009,424. The top of refractory mass 4 isjust below the level of the top 10 of the carbon bars 5, and the blocks34 are placed alongside the bars 5 whereby they provide a recess 36which is filled with molten aluminum 6. The walls of the recess 36 canbe sloping, as shown, or vertical. Thus, the molten aluminum 6 forms ashallow pool or film about 3-30 mm thick above the aluminum-wettablesurface of the RHM upper layer 35, but forms a deeper pool, e.g., about25-60 mm thick, in the recesses 36 above the top 10 of the carbon bars5, which protects the carbon from attack by the electrolyte. Above themolten aluminum 6 is a layer of molten electrolyte 7 in which the anodes9 dip. Typically two rows of anodes 9 are arranged side-by-side with anysuitable number of anodes along the cell length according to the cellcapacity. Advantageously, as shown, the anodes 9 will be non consumableoxygen-evolving anodes, e.g., coated with a cerium oxide-fluoridecoating 39. A trough or other arrangement, not shown, is provided at thesides and/or ends of the cell for containing and tapping off theproduced aluminum.

We claim:
 1. A cell for the electrowinning of aluminum from molten saltshaving a plurality of anodes disposed over a cell bottom comprising acarbon cathode through which current is supplied to a pool of moltenaluminum on the cell bottom, characterized in that the cell bottom islined with a plurality of sections of carbon and at least one mass ofnon-conductive, refractory material juxtaposed with the carbon sectionsto make up a composite cell bottom composed of adjacent areas ofcurrent-conducting carbon and non-conducting refractory material, theupper surfaces of the carbon section areas being located at the samelevel as or lower than the upper surfaces of the refractory areas, andthe total upwardly facing surface area of the carbon sections in thecell bottom located under the anodes being smaller than the horizontalsurface area of the anodes, with the carbon section areas being placedout of correspondence with the cathodes.
 2. A cell according to claim 1,in which the refractory mass occupies at least 30% of the surface areaof the cell bottom.
 3. A cell according to claim 1, in which therefractory mass extends to the cell sides.
 4. A cell according to claim1, in which the refractory mass comprises tabular alumina.
 5. A cellaccording to any of claim 1, in which at least part of the refractorymass consists of fused alumina.
 6. A cell according to claim 1, in whichthe surface of the refractory mass is wettable by molten aluminum.
 7. Acell according to claim 1, in which the level of the refractory mass isthe same as the level of the carbon cathode.
 8. A cell according toclaim 1, in which the level of the refractory mass is higher than thelevel of the carbon cathode.
 9. A cell according to claim 8, in whichthe pool of aluminum above the refractory mass has a minimum level abovethe carbon cathode such that the level of molten aluminum maintainedpermanently is sufficient to protect the carbon from contact with theelectrolyte during fluctuations of the pool level above the refractorymass.
 10. A cell according to claim 1, in which the carbon cathodesections are longitudinal in the cell.
 11. A cell according to claim 1,in which the carbon cathode sections are transversal in the cell.
 12. Acell according to claim 1, in which the electrical contact of the carboncathode to external bus bars is made through collector bars extendinghorizontally through the cell bottom.
 13. A cell according to claim 12,in which vertical pins, plates or bars of metal resisting the operatingtemperature of the cell are inserted in the carbon cathode and connectedto the collector bars.
 14. A cell according to claim 1, in whichvertical pins, plates or bars of metal resisting the operatingtemperature of the cell are inserted in the carbon cathode and connectedto the cell outside shell and from there to the bus bars.
 15. A cellaccording to claim 1, in which the surface of the carbon cathodes incontact with the molten aluminum is increased to improve electricalcontact by providing cuts, holes, slots or other recesses in the carbonbody extending vertically but not reaching the current collecting means,these cuts, holes, slots or recesses being filled with molten aluminum.16. A cell according to claim 1, in which spacings are provided betweenthe carbon cathodes and the adjacent refractory mass, these spacingsextending vertically and being filled with molten aluminum, but notreaching the current collecting means.
 17. A cell according to claim 1,in which the anodes are oxygen evolving anodes.
 18. A cell according toclaim 17, in which the anodes are dimensionally stable.
 19. A cell forthe electrowinning of aluminum from molten salts having a plurality ofanodes disposed over a cell bottom comprising a carbon cathode throughwhich current is supplied to a pool of molten aluminum on the cellbottom, characterized in that the cell bottom is lined with at least onebody of carbon and at least one mass of non-conductive, refractorymaterial juxtaposed with the carbon body or bodies to make up acomposite cell bottom composed of adjacent areas of current-conductingcarbon and non-conducting refractory material, the upper surfaces of thecarbon areas being located at the same level as or lower than the uppersurfaces of the refractory areas, with the total upwardly facing surfacearea of the carbon in the cell bottom located under the anodes beingsmaller than the horizontal surface area of the anodes, and with theelectrical contact of the carbon cathode to external bus bars being madethrough collector bars extending horizontally through the cell bottom,while vertical pins, plates or bars of metal resisting the operatingtemperature of the cell are inserted in the carbon cathode and connectedto the collector bars.
 20. A cell for the electrowinning of aluminumfrom molten salts having a plurality of anodes disposed over a cellbottom comprising a carbon cathode through which current is supplied toa pool of molten aluminum on the cell bottom, characterized in that thecell bottom is lined with at least one body of carbon and at least onemass of non-conductive, refractory material juxtaposed with the carbonbody or bodies to make up a composite cell bottom composed of adjacentareas of current-conducting carbon and non-conducting refractorymaterial, the upper surfaces of the carbon areas being located at thesame level as or lower than the upper surfaces of the refractory areas,with the total upwardly facing surface area of the carbon in the cellbottom located under the anodes being smaller than the horizontalsurface area of the anodes, and with vertical pins, plates or bars ofmetal resisting the operating temperature of the cell being inserted inthe carbon cathode and connected to a cell outside shell and from thereto bus bars.
 21. A cell for the electrowinning of aluminum from moltensalts having a plurality of anodes disposed over a cell bottomcomprising a carbon cathode through which current is supplied to a poolof molten aluminum on the cell bottom, characterized in that the cellbottom is lined with at least one body of carbon and at least one massof non-conductive, refractory material juxtaposed with the carbon bodyor bodies to make up a composite cell bottom composed of adjacent areasof current-conducting carbon and non-conducting refractory material, theupper surfaces of the carbon areas being located at the same level as orlower than the upper surfaces of the refractory areas, with the totalupwardly facing surface area of the carbon in the cell bottom locatedunder the anodes being smaller than the horizontal surface area of theanodes, and with the surface of the carbon cathode in contact with themolten aluminum being increased to improve electrical contact byproviding cuts, holes, slots or other recesses extending vertically butnot reaching current collecting means, these cuts, holes, slots orrecesses being filled with molten aluminum.
 22. A cell according toclaim 21, in which said recesses are provided between the carboncathodes and the adjacent refractory mass, these recesses extendingvertically and being filled with molten aluminum, but not reaching thecurrent collecting means.
 23. A cell according to claim 19, 20 or 21, inwhich the carbon cathode consists of a plurality of sections and thecarbon cathode sections in the cell are placed under the anodes, or thecarbon cathode sections in the cell are placed out of correspondencewith the anodes.
 24. A method of electrowinning aluminum from a moltensalt in a cell having a plurality of anodes disposed over a cell bottomcomprising a carbon cathode through which current is supplied to a poolof molten aluminum on the cell bottom, said method being characterizedby lining the cell bottom with a plurality of sections of carbon and atleast one mass of non-conductive, refractory material juxtaposed withthe carbon sections to make up a composite cell bottom composed ofadjacent areas of current-conducting carbon and non-conductingrefractory material, positioning the upper surfaces of the carbonsections at the same level as or lower than the upper surfaces of therefractory areas, supplying sufficient of said carbon sections toprovide a total upwardly facing surface area of the carbon sections inthe cell bottom that is smaller than the horizontal surface area of theoverlying anodes, and passing current from said carbon sections ofsmaller surface area.
 25. The method of claim 24, wherein said currentis passed in a cell renovated by replacing some used blocks of carbonwith new blocks of carbon, while replacing some used blocks of carbonwith said non-conductive, refractory material.
 26. A cell for theelectrowinning of aluminum from molten salts having a plurality ofanodes disposed over a cell bottom comprising a carbon cathode throughwhich current is supplied to a pool of molten aluminum on the cellbottom, characterized in that the cell bottom is lined with a pluralityof sections of carbon extending throughout the thickness of the cellbottom and forming the cathode, with said cell bottom having at leastone mass of non-conductive, refractory material, juxtaposed with thecarbon sections and extending throughout the thickness of said cellbottom, said carbon sections and said juxtaposed refractory materialforming a composite cell bottom composed of adjacent areas ofcurrent-conducting carbon and non-conducting refractory material, theupper surfaces of the carbon sections being located at the same level asor lower than the upper surfaces of the refractory areas, with the totalupwardly facing surface area of the carbon in the cell bottom beingsmaller than the horizontal surface area of the anodes, and with thecarbon section areas being placed out of correspondence with theoverlying anodes.